As feature sizes continue to decrease in modern photolithographic semiconductor manufacturing processes effects, such as undesired line edge roughness, insufficient lithographical resolution, and limited depth of focus problems, may increase. Specifically, photoresist image footprints typically become increasingly difficult to control as semiconductor device features become smaller and closer together, because these undesired effects seldom scale with the reduced feature size. More specifically, the molecular size in the polymer backbone used in the photoresist must typically meet certain physical/mechanical requirements and, therefore, should not be scaled with the reduction in feature size.
Line edge roughness is typically at least partially caused by acid diffusion that results when a photoresist is exposed to an incident radiation, such as ultra-violet light. The acid typically serves as a catalyst for chemical reactions (deprotection) that occur within the photoresist at high temperatures, such as a post-exposure bake. The acid may be produced by photoacid generator molecules that are blended into the photoresist and diffuse the acid upon exposure to incident radiation.
FIG. 1 is a photograph illustrating the effects of line edge roughness in a semiconductor device feature. Note that the line edge roughness results in a non-uniform feature line edge, thus limiting the feature size of the device.
Prior art methods for controlling line edge roughness include adding a base quencher to the photoresist that has a relatively low base concentration with respect to the acid concentration. Acid concentrations less than or equal to the base concentration would be quenched (neutralized) by the base.
During a post-exposure bake, at least some of the acid diffusion at a feature line edge formed by the exposed and developed photoresist would be neutralized by the base. This results in a better-defined feature edge and reduced line edge roughness. Using a base quencher prior to exposing and developing the photoresist, however, can result in a decrease in photoresist sensitivity, thereby increasing the incident radiation energy necessary to produce the same result.